My understanding is that 50% is the maximum aggregate that a player's salary can be retained. I don't think one team can retain 50% and then a second team retain another 25%.

Thats's the way I interpreted the rule.

I re-read the FAQ on here and you might be right. I was thinking it was the team couldn’t retain more than 50% but it looks like a contract can’t have more than 50% retained.

I don't believe that is the case.
The aggregate cannot be over 15% of the cap
A contract can be subjected to a maximum of 2 retained transactions.
The acquiring team just must take on at least 50% of the "current" value of the contract

I believe it happened with a goaltender a few years ago

I did the same and then looked it up in the CBA as well.

Are you sure? I'd rather be corrected and get it right.

I think there was someone that was traded twice in quick succession with retained salary but idk if it exceeded 50%.